Hood won't latch - what to check?

[Mitch528i]

Help! I popped the hood on my 1997 528i yesterday to fill the washer fluid. . .when I closed it, it won't latch. The hook-shaped safety release stays extended unless I push down HARD on the hood. Even when it looks like it's latched, I can pop it up by pulling up on the hood.

I was thinking something must be frozen, since it was 9 degrees when it happened. Can someone educate me on the workings of the hood latch on this car? I can't seem to figure out where to begin troubleshooting something like this.

Thanks!

PS - I keep thinking if the BMW washer fluid reservoir held more than 6 ounces of fluid, this wouldn't have happened!